Maxxair screws to the side of your vent, not the roof, and they are great. You can leave vent open when it rains.
If you are really inventive, and your vent is not powered, you can screw, or pop rivet, a 12v. computer muffin type fan to the inside strut on the vent. Of course you would have to get power to it. I used a small solar panel, and hid the wire.
Just my tuppence, (British for 2 pennies.) Penny